DRAFT
March 2012
MAMMALS Pallid Bat (Antrozous pallidus)
Pallid Bat
(Antrozous pallidus)
Legal Status
State: Species of Special Concern
Federal: Bureau of Land Management Sensitive
Critical Habitat: N/A
Recovery Planning: N/A
Notes: None
Taxonomy
The pallid bat (Antrozous pallidus) is the only species in the genus Antrozous of the family Vespertilionidae (Hermanson and O’Shea 1983; Hoofer et al. 2003) (Antrozous formerly included A. dubiaquercus, but this Central American species is now assigned to the genus Bauerus [Hermanson and O’Shea 1983]). A study of phylogenetic relationships of plecotine bats using mitochrondrial ribosomal sequences supported the placement of pallid bat as a single-species genus in the family Vespertilionidae (Hoofer et al. 2003). There are seven recognized subspecies of pallid bat (Wilson and Reeder 2005), of which A. p. pallidus is likely the subspecies present in the Desert Renewable Energy Conservation Plan (DRECP) project Plan Area, although A. p. pacificus may also occur in the western portion of the Plan Area (Hall 1981). The status of pallid bat as California Species of Special Concern is for the full species A. pallidus, so a subspecific assignment is not relevant to the conservation of this species in the Plan Area. No other available information indicates other important taxonomic considerations. The species’ physical characteristics are described in detail in Hermanson and O’Shea (1983).
Distribution
General
The pallid bat is widespread throughout the western United States; southern British Columbia, Canada; and mainland and Baja California, Mexico (Hermanson and O'Shea 1983; Hall1981). Within the United States, it ranges east into southern Nebraska, western Oklahoma, and western Texas (Figure SP-M6). The pallid bat is locally common in the Great Basin, Mojave, and Sonoran deserts (especially the Sonoran life zone) and grasslands throughout the western United States, and it also occurs in shrublands, woodlands, and forests at elevations up to 2,440 meters (8,000 feet) (Hermanson and O'Shea 1983; Hall 1981). The pallid bat occurs throughout California, except at the highest elevations of the Sierra Nevada range. Although this species prefers rocky outcrops, cliffs, and crevices with access to open habitats for foraging, it has been observed far from such areas (Hermanson and O'Shea 1983).
Distribution and Occurrences within the Plan Area
Historical
The DRECP database for pallid bat, composed of Bureau of Land Management (BLM) and California Natural Diversity Database (CNDDB) (CDFG 2011) records, and observations by Brown (CDFG 2011; Dudek 2011), includes 20 historical records (i.e., pre-1990) for the Plan Area, dating from 1911 to 1981, and two with an unknown observation date. An additional 11 records are from areas within 5 miles of the Plan Area boundary. The historical occurrences in the Plan Area include the southern Owens Valley–eastern Sierra Nevada–Inyo Mountains area, the Mesquite Mountains in eastern San Bernardino County, the Twentynine Palms area, the lower Colorado River, and the Salton Sea area.
See Figure SP-M6 for historical and recent occurrences of pallid bat in the Plan Area.
Recent
There are 38 recent (i.e., since 1990) records in the Plan Area and 10 additional records within the 5-mile buffer area around the Plan Area (CDFG 2011; Dudek 2011). The geographic areas of recent occurrences are similar to the historical occurrences, with small clusters of observation in the Owens Valley–eastern Sierra Nevada area, Providence Mountains, Kingston Range, Avawatz Mountains, Cady Mountains, Twentynine Palms area, Little San Bernardino Mountains, Hexie Mountains, the Lower Colorado River, Chocolate Mountains, and the Peninsular Range in east San Diego County.
As with the historical data, the specificity of these recent occurrence data is variable, with some records identifying roosts and others only including general location information for observations. This dataset, therefore, should be viewed as reflecting the recent documented distribution of the species in the Plan Area and should not be used as detailed data for specific roost sites.
Natural History
Habitat Requirements
Pallid bat day roosting habitat typically includes rocky outcrops, cliffs, and spacious crevices with access to open habitats for foraging (Hermanson and O'Shea 1983; Vaughan and O’Shea 1976). Pallid bats may also roost in caves, mines, bridges, barns, porches, and bat boxes, and even on the ground under burlap sacks, stone piles, rags, baseboards, and rocks (Beck and Rudd 1960; Rambaldini 2006). Up to the late 1940s, they were common in buildings at low elevations of the South Coast Ecoregion (Miner and Stokes 2005). For example, in the Newhall area of Southern California, they recently were observed using buildings for both day and night roosts (Johnson 2006). In Northern California, they were observed using buildings and large-diameter, tall, live trees and snags in mature forest stands for both day and night roosting (Baker et al. 2008). In Baker et al. (2008), live trees and snags used for roosting were consistently tall in height, large in diameter, and located in mature stands in micro-sites with low percentages of overstory and mid-story cover. Day roosts generally are warm, have obstructed entrances and exits, and are high enough to avoid terrestrial predators (Rambaldini 2006). A study of night roosts, including rock overhangs, bridges, and buildings, in Oregon found that they were protected from rain and allowed free flight space for bats in and out of the roost (Lewis 1994).
Although pallid bats may use a variety of roosting habitats, they are also selective of roost sites with microenvironments that minimize energy expenditure through adaptive hypothermia and maintain low metabolic rates (Vaughan and O’Shea 1976). In spring and fall at roost sites in Central Arizona, they used vertical crevices that passively warmed during the afternoon prior to emergence, and in the summer, they used deep horizontal crevices that acted as heat sinks and kept ambient temperatures low (Vaughan and O’Shea 1976). A roost temperature of about 30 degrees Celsius (86 degrees Fahrenheit) is considered about optimal for maintaining low metabolic rates (Trune and Slobodchikof 1976; Vaughan and O’Shea 1976). In desert regions, roost sites are often near water, although they have been observed in areas without apparent water sources (Hermanson and O’Shea 1983).
Pallid bat day roosts consisting of single- or mixed-sex colonies usually are established in crevices or man-made structures. Day roosts usually have at least 20 individuals and sometimes more than 200 individuals (Hermanson and O'Shea 1983).
Foraging habitats for pallid bats are varied and include grasslands, oak savannah woodlands, open pine forests, talus slopes, and agricultural areas (Rambaldini 2006). In a study of bat use of riparian habitats in southern Nevada, including riparian marsh, mesquite bosque, riparian woodland, and riparian shrubland, Williams et al. (2006) recorded about 88% of pallid bat occurrences in riparian woodland. Although most foraging probably occurs in close proximity to night roosts, movements greater the 2 kilometers (1.2 miles) from roosting sites in forest habitats are common (Baker et al. 2008), and movements up to 30 kilometers (18.6 miles) have been recorded (Hermanson and O’Shea 1983). See discussion in Spatial Behavior for more information.
Table 1 summarizes the likely habitat associations for pallid bat in the Plan Area.
Table 1. Habitat Associations for Pallid Bat
Land Cover Type / Land Cover Use / Habitat Designation / Habitat Parameters / Supporting Information /Rocky, Barren, and Unvegetated Community / Day and night roosts / Day and night roosting / 50% rocky slopes within 6.2 miles of water source / Hermanson and O’Shea 1983
All natural land covers (i.e., except developed and disturbed) / Foraging / Primary foraging / Natural land covers within 3.1 miles of day roosting habitat / Baker et al. 2008; Bell 1982; Rambaldini 2006
All natural land covers (i.e., except developed and disturbed) / Foraging / Secondary foraging / Natural land Covers 3.1 to 6.2 miles of day roosting habitat / Baker et al. 2008; Bell 1982; Rambaldini 2006
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Notes: Water sources include major rivers, reservoirs, lakes, ponds, seeps and springs, and perennial streams. Pallid bats are expected to forage in virtually all relatively open, natural land covers in the Plan Area where suitable prey are present.
Foraging Requirements
Pallid bats forage about 0.5 to 2.5 meters [1.6 to 8.2 feet] above the ground surface, and their foraging behavior is directed toward prey that are close to the ground, on the ground, or perched on exposed vegetation (O’Shea and Vaughan 1977). They may forage both aerially and by gleaning from plants, and they have also been observed to take prey by crawling along the ground. Their diet generally has been described to include scorpions, ground crickets, solpugids, darkling ground beetles, carrion beetles, short-horned grasshoppers, cicadas, praying mantids, long-horned beetles, and sphingid moths (Hermanson and O’Shea 1983). While pallid bats are primarily insectivores, they have also been observed to eat lizards and smaller bats in captivity (Hermanson and O’Shea 1983) and likely take a variety of small vertebrates in the wild. Their specific diets vary geographically and may reflect genotypic or phenotypic selection (Johnston and Fenton 2001). Pallid bats generally take large prey (up to 6.0 centimeters [2.4 inches] total body length) (O'Shea and Vaughan 1977). In both a coastal area (Marin County) and a desert area (Caliente Mine in Death Valley) in California, pallid bats foraged for Orthoptera (grasshoppers, crickets) and Coleoptera (beetles), and smaller percentages of Solpugida (sun scorpions), Lepidoptera (moths), and Diptera (flies). At Caliente Mine, Coleoptera made up about 55% of their diet by volume, but diet changed over time, reflecting the availability of prey. Individuals in the local population tended to have the same diet at any given time (Johnston and Fenton 2001). In contrast, at the Marin County site, diets were varied, but the variation was related to individual differences (i.e., there was no “average” diet for the group such as that of the Caliente site), and these differences may have reflected learning that reduces searching and handling time (Johnston and Fenton 2001).
Reproduction
Pallid bats breed in October through December, and possibly through February (Hermanson and O’Shea 1983) (Table 2). Females store sperm and ovulation occurs during the following spring. Gestation is approximately 9 weeks, and birth in the southwestern United States typically occurs from May through June (Hermanson and O’Shea 1983). Litter size is typically 2 young (approximately 80% of litters (Bassett 1984)), and occasionally 3; yearling females may breed but litter size is 1 (Davis 1969; Hermanson and O’Shea 1983). The young are born relatively undeveloped, but they mature rapidly and engage in their first flight at 33 to 36 days (Davis 1969). They achieve full adult flight capability by about 49 days of age and full adult weight by 56 days of age (Hermanson and O’Shea 1983). Yearling males are not sexually active their first autumn and probably not their first year (Davis 1969). Mature males and females have the same body dimensions (e.g., weight, forearm length, wing area); i.e., they do not exhibit sexual dimorphism (Davis 1969).
Pallid bats have lived up to 9 years in captivity (Hermanson and O’Shea 1983).
Table 2. Key Seasonal Periods for Pallid Bat
/ Jan / Feb / March / April / May / June / July / Aug / Sep / Oct / Nov / Dec /Breeding / ? / ? / x / x / x
Birth/
Development / x / x / x / x
Winter Torpor / x / x / x / x / x
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Sources: Bassett 1984; Davis 1969; Hermanson and O’Shea 1983.
Spatial Behavior
Pallid bats in central Arizona exhibited a bimodal foraging activity pattern, with two foraging bouts separated by a period of night roosting, with the timing and duration of these activities seasonally variable (O’Shea and Vaughan 1977). During the summer, time away from the day roost varied from approximately 45% to 58% of the night and 25% to 27% in September and October (O’Shea and Vaughan 1977). Pallid bats may be active outside the roost any time of year, but their activity during the winter may be erratic, which probably is associated with cold periods when they are in torpor (Table 2). They have been mist-netted at temperatures as low as 2 degrees Centigrade (35.6 degrees Fahrenheit) in southern Nevada (O’Farrell et al. 1967). In contrast to O’Shea and Vaughan (1977), O’Farrell et al. (1967) did not detect a bimodal activity period in southern Nevada during the fall and winter; all captures were 1.5 to 5 hours after dusk. This information indicates that nightly foraging activity by pallid bats is seasonally variable.
During July through August, pallid bats in central Arizona showed little fidelity to specific roosting sites, but during the cooler months they showed greater fidelity to certain roosting sites (O’Shea and Vaughan 1977), which may reflect more specific roost requirements during the colder months to maintain thermoregulation (also see Habitat Requirements regarding day roost characteristics).
The distances that pallid bats travel during foraging bouts may be limited by the availability of night roosts because they frequently bring large prey to these sites where they are then eaten (O’Shea and Vaughan 1977). Bell (1982), for example, observed pallid bats foraging within 3 kilometers (1.9 miles) of roost sites in desert grasslands in New Mexico. A radio-tracking study in British Columbia found that foraging occurred within 1.5 kilometers (0.9 mile) of day roost sites (Rambaldini 2006). In this study, males returned to the day roost for short periods between foraging bouts (Rambaldini 2006) (however, note from discussion above that nightly foraging activity is seasonally variable). In coniferous forest in Northern California, radio-tracking documented that foraging bouts more than 2 kilometers (1.2 miles) from the day roost were common, but most foraging occurred in close proximity to day roosts (Baker et al. 2008). The longest distance moved during this study was 4.7 kilometers (2.9 miles) by a pregnant female. Lactating females had average foraging ranges of 1.56 square kilometers (0.6 square mile), and post-lactating females had average ranges of 5.97 square kilometers (2.3 square miles) (Baker et al. 2008). However, flights up to 30 kilometers (19 miles) between night roosts have been recorded, indicating that pallid bats have the capacity to fly long distances. Further, homing studies have shown a maximum return distance of 174 kilometers (108 miles), and several recoveries have shown return distances of 48 to 51 kilometers (30 to 32 miles) from release sites within 7 to 8 hours after release (Hermanson and O’Shea 1983).